Static Var Compensator (SVC) systems have been used in power networks for many years for controlling generation and absorption of reactive power in a power network in order to provide voltage support, improve transient stability and to damp oscillations. A STATCOM (STATic COMpensator) is a kind of a SVC, being a Voltage Source Converter (VSC) based device that can act as either a source or sink of reactive AC power to the power network. The STATCOM can also provide active power if it is connected to a DC power source.
FIG. 1 illustrates a prior art two-level static compensator 1 without any transformers to step down the power network voltage. The static compensator 1 comprises a voltage source converter (VSC) 2 connected at its DC side to a capacitor 3 and at its AC-side to a power network 8, also denoted grid.
The conventional two-level VSC 2 comprises three phase-legs P1, P2, P3, each phase-leg consisting of two series-connected valves. The two valves of phase-leg P1 are indicated at reference numerals 9a, 9b. Each valve 9a, 9b in turn comprises a transistor with an anti-parallel diode, or rather, in order to manage high voltages, each valve comprises a number of series-connected transistors, for example Insulated Gate Bipolar Transistors (IGBTs), each IGBT having an anti-parallel diode.
The VSC 2 is connected to the grid 8, in FIG. 1 comprising a three phase network, via a phase reactor 4, via a starting resistor 5 connected in parallel with a switch 6 and via an AC circuit breaker 7 in each phase. Each phase, or at least two of them, comprises such phase reactor, starting resistor, switch and circuit breaker. The respective phases are connected to the middle point of the respective phase-leg P1, P2, P3, i.e. connected between the respective valves as illustrated in the figure. It is possible to reduce the number of components by equipping only two of the phases with the starting resistor connected in parallel with the switch. Only one phase is described in the following in order to simplify the description, but it is understood that the phases are similar.
When the grid-connected VSC 2 is to be energized and started, the circuit breaker 7 is switched so as to provide a current path from the grid 8 through the starting resistor 5, the phase reactor 4, and through the diodes of the VSC 2 so as to charge the capacitor 3. When the capacitor voltage has reached a predetermined level, the starting resistor 5 is short-circuited by closing the parallel-connected switch 6. As the starting resistor 5 is short-circuited, the capacitor voltage will increase a bit more and when it is high enough, the valves of the VSC 2 are deblocked and start to switch. The capacitor voltage is then controlled up to its reference value.
The starting resistor 5 is provided in order to protect the diodes of the VSC 2 from being damaged by a too high and/or too long-lasting current surge, which could occur upon closing the AC circuit breaker 7 without the use of the starting resistor 5.
The stress put on the valves, and in particular the diodes, of the VSC 2 depend on several factors, for example the size of the DC-side capacitor 3, the size of the phase reactors 4 and on the voltage levels of the power network 8.
A chain-link converter comprises a number of series-connected cells, each cell comprising a capacitor, besides the valves. The DC-capacitor of each such cell is rather big compared to the above described two-level static compensator 1, when seen in relation to the total effect of the system. Having a large capacitor entails having a rather large current through the diodes during a relatively long duration before the capacitor is charged enough to start switching the valves of the chain-link converter. This in turn entails the risk of the diodes being damaged during the start-up of the converter.
Although the starting resistors and switches provide a functioning solution, it is a rather expensive solution and it would be desirable to lower the costs of the converter. It is conceivable to use diodes able to handle larger currents, but this is still an expensive solution.